Canon LBP-2000 Service Manual page 55

CHAPTER 2
4 . Protective functions
The printer is equipped with the following protective functions to prevent malfunction of the fixing
h e a t e r :
a . Thermistor (detecting overheating)
The ASIC and the high-temperature detection circuit found on the DC controller PCB monitors the
voltage readings of the main thermistor and the sub thermistor; if the main thermistor detects a
voltage level which is equivalent of the following temperature, the CPU will identify the condition
as being overheating:
• 220°C or higher (by main thermistor TH101)
b . Thermistor (detecting connector disconnection)
The connector disconnection detection circuit found on the DC controller PCB monitors the THCNCT
signal; if the THCNCT signal goes Low, the CPU will identify the condition as indicating disconnec-
tion of the thermistor connector.
c . Thermistor (cutting the power for overheating)
If the internal temperature of the thermal switch (THSW101) exceeds about 230°C, thermal switch
will go OFF, thereby cutting the power to the fixing heater.
d . Detecting the activation of the fixing heater
The fixing heater activation detection circuit found on the power supply PCB assumes that the /
HEATDT signal is Low if the /FSRDRV signal is Low (fixing heater ON). If /HEATDT goes High
while /FSRDRV signal is Low (fixing heater ON), the ASIC will identify the condition to be an error
in the activation of the fixing heater (triac short circuit).
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